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<title>Constraints (GNU Compiler Collection (GCC) Internals)</title>

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Next: <a href="Standard-Names.html" accesskey="n" rel="next">Standard Pattern Names For Generation</a>, Previous: <a href="Predicates.html" accesskey="p" rel="prev">Predicates</a>, Up: <a href="Machine-Desc.html" accesskey="u" rel="up">Machine Descriptions</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html" title="Index" rel="index">Index</a>]</p>
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<h3 class="section" id="Operand-Constraints"><span>17.8 Operand Constraints<a class="copiable-link" href="#Operand-Constraints"> &para;</a></span></h3>
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<p>Each <code class="code">match_operand</code> in an instruction pattern can specify
constraints for the operands allowed.  The constraints allow you to
fine-tune matching within the set of operands allowed by the
predicate.
</p>
<p>Constraints can say whether
an operand may be in a register, and which kinds of register; whether the
operand can be a memory reference, and which kinds of address; whether the
operand may be an immediate constant, and which possible values it may
have.  Constraints can also require two operands to match.
Side-effects aren&rsquo;t allowed in operands of inline <code class="code">asm</code>, unless
&lsquo;<samp class="samp">&lt;</samp>&rsquo; or &lsquo;<samp class="samp">&gt;</samp>&rsquo; constraints are used, because there is no guarantee
that the side effects will happen exactly once in an instruction that can update
the addressing register.
</p>


<ul class="mini-toc">
<li><a href="Simple-Constraints.html" accesskey="1">Simple Constraints</a></li>
<li><a href="Multi_002dAlternative.html" accesskey="2">Multiple Alternative Constraints</a></li>
<li><a href="Class-Preferences.html" accesskey="3">Register Class Preferences</a></li>
<li><a href="Modifiers.html" accesskey="4">Constraint Modifier Characters</a></li>
<li><a href="Machine-Constraints.html" accesskey="5">Constraints for Particular Machines</a></li>
<li><a href="Disable-Insn-Alternatives.html" accesskey="6">Disable insn alternatives using the <code class="code">enabled</code> attribute</a></li>
<li><a href="Define-Constraints.html" accesskey="7">Defining Machine-Specific Constraints</a></li>
<li><a href="C-Constraint-Interface.html" accesskey="8">Testing constraints from C</a></li>
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